A microscope which automatically follows the motion of individual cells is being used to characterize the photoresponses of Halobacterium halobium, a bacterium which contains a pigment, bacteriorhodopsin, which is similar to rhodopsin, the visual pigment used by man. There are two photoresponses: in one, the cells back up when the intensity of orange light is decreased. The second response involves rhodopsin. Its molecular basis is being analyzed through the study of behavioral mutants.